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United States Patent |
5,337,355
|
Tomasini
,   et al.
|
August 9, 1994
|
Supply circuit device for a user's telephone circuit, having a low
voltage loss
Abstract
A circuit is described which comprises an operational amplifier, two
resistors connected between the telephone line and the inputs of the
amplifier, a capacitor which is charged via a first bipolar transistor
controlled by the amplifier via a first FET transistor, a second bipolar
transistor in parallel to the connection of the first transistor and the
capacitor, a second FET transistor, identical to the first and having its
source and gate terminals connected to the corresponding terminals of the
first, and two current generators connected to the drain terminals
respectively of the first and the second FET transistor and to the bases,
respectively, of the first and second bipolar transistor. The currents of
the two generators and the other parameters of the circuit are such as to
hold the first and second bipolar transistors in a conductive and a
switched off state respectively, except when the line voltage fails below
a minimum predetermined value; in which case the first and second
transistors commute to the switched off and conductive states
respectively. The circuit has a lower "voltage loss" than known circuits.
Inventors:
|
Tomasini; Luciano (Monza, IT);
Castello; Rinaldo (Arcore, IT)
|
Assignee:
|
SGS-Thomson Microelectronics S.r.l. (Milan, IT)
|
Appl. No.:
|
991564 |
Filed:
|
December 16, 1992 |
Current U.S. Class: |
379/413; 379/324; 379/395.01 |
Intern'l Class: |
H04M 019/00; H04M 001/00 |
Field of Search: |
379/413,324,399,400,401,387
|
References Cited
U.S. Patent Documents
4628249 | Dec., 1986 | Ikefuji et al. | 323/315.
|
4639551 | Jan., 1987 | Kaire | 379/413.
|
4811391 | Mar., 1989 | Van Dongen et al. | 379/387.
|
Foreign Patent Documents |
2538661 | Jun., 1984 | FR | .
|
2549667 | Jan., 1985 | FR | .
|
Primary Examiner: Dwyer; James L.
Assistant Examiner: Fournier; Paul A.
Attorney, Agent or Firm: Seed and Berry
Claims
We claim:
1. A circuit arrangement for drawing a supply voltage from a user's
telephone line, particularly for supplying a speech circuit, comprising:
an operational amplifier having an output, an inverting input and a
non-inverting input,
resistive means between a wire of the telephone line and the inputs of the
operational amplifier,
negative feedback means between the output of the operational amplifier and
a circuit node connected to one of the inputs of the operational
amplifier,
coupling means adapted to apply to the other of the inputs of the
operational amplifier a current proportional to the line current,
a capacitor connected on one side to a common terminal of the circuit
arrangement which is connected to the other wire of the telephone line
and, on the other side, to an output terminal of the circuit arrangement,
which is connected to the said circuit node by means of first circuit
means controlled by the operational amplifier,
characterized in that such first circuit means include a first transistor
device having a conduction path connected in series with the capacitor and
a control terminal, and in that it further comprises:
a second transistor device having a respective conduction path connected in
parallel to the series connection of the first transistor device with the
capacitor, and a respective control terminal, and
third and fourth transistor devices each having a conduction path
connected, on one side to the common terminal and, on the other side to
the circuit node via first and second current generators respectively, and
a control terminal, these control terminals being connected together to
the output of the operational amplifier;
the node between the third transistor device and the respective first
current generator being connected to the control terminal of the first
transistor device and the node between the fourth transistor device and
the respective second current generator being connected to the control
terminal of the second transistor device, and the circuit parameters being
chosen in such a way that the first and the second transistor device are,
respectively, in a conduction state and a switched-off state except when
the line voltage falls below a minimum predetermined value, in which case
the first transistor device is switched off and the second transistor
device is conductive.
2. A circuit device according to claim 1, characterized in that the first
and second transistor devices are each constituted by a pair of bipolar
transistors in a Darlington configuration.
3. A circuit device according to claim 1 or claim 2, characterized in that
the third and fourth transistor devices are each constituted by a
field-effect transistor.
4. A circuit device according to claim 3, characterized by the fact that
the two field-effect transistors are identical and by the fact that the
first current generator generates a current less than the current of the
second current generator.
5. A circuit arrangement according to claim 1 characterized by the fact
that the first and second current generators each comprise a pair of
bipolar transistors connected in current mirror configuration and a
respective current generator.
6. A circuit for providing a supply voltage from a telephone exchange
having first an d second supply wires to a user's telephone with a minimum
voltage loss, the circuit comprising:
an operational amplifier having non-inverting and inverting input terminals
coupled to the first supply wire through the first and second resistances,
respectively, and a differential output terminal producing an output
voltage signal proportional to the difference between the voltage signals
of said first and second input terminals;
a current generator coupled between said first resistance and the second
supply wire to generate a current;
a first output transistor coupled to a capacitor and said second resistance
and controlled by a first control current, said capacitor being coupled to
the circuit output to produce a voltage for the user's telephone;
a second output transistor coupled to the second supply wire and said
second resistance and controlled by a second control current;
first and second constant current generators coupled to said second
resistance and producing first and second constant currents, said second
constant current being greater than said first constant current; and
first and second identical control transistors, controlled by said output
voltage signal and coupled to said first and second constant current
generators, respectively, said first and second control transistors each
conducting an identical current from said first and second constant
current generators to the second supply wire, wherein a node coupling said
first constant current generator to said first control transistor also
produces said first control current, wherein a node coupling said second
constant current generator to said second control transistor also produces
said second control current, whereby said first output transistor is
conducting and said second output transistor is cutoff when the voltage on
the first supply wire is above a predetermined threshold, and said first
output transistor is cutoff and said second output transistor is
conducting when the voltage on the first supply wire is below said
predetermined threshold.
7. The circuit of claim 6 wherein said second constant current has a value
equal to the sum of said first constant current and the maximum value for
said first control current.
8. The circuit of claim 6 wherein said first output transistor operates in
a linear mode of operation if the voltage on the first supply wire is
above a second threshold.
9. The circuit of claim 6 wherein said capacitor charges with a current
whose value is determined by the ratio between said first and second
resistances.
10. The circuit of claim 8 wherein said first output transistor operates in
a saturated mode of operation if the voltage on the first supply wire is
below said second threshold.
11. The circuit of claim 10 wherein said capacitor charges with a current
determined by the current conducted by said first output transistor and is
proportional to the ratio between said first and second resistors.
12. The circuit of claim 6 wherein said first and second output transistors
are bipolar transistors.
13. The circuit of claim 6 wherein said first and second output transistors
are Darlington transistors.
14. The circuit of claim 6 wherein said first and second control
transistors are identical field-effect transistors.
15. A circuit for providing a supply voltage from a telephone exchange
having first and second supply wires to a user's telephone with a minimum
voltage loss, the circuit comprising:
an operational amplifier having non-inverting and inverting input terminals
coupled to the first supply wire through first and second resistances,
respectively, and a differential output terminal producing an output
voltage signal proportional to the difference between the voltage signals
of said first and second input terminals;
a current generator coupled between said first resistance and the second
supply wire to generate a current;
a first output transistor coupled to a capacitor and said second resistance
and controlled by a first control current, said capacitor being coupled to
the circuit output to produce a voltage for the user's telephone;
a second output transistor coupled to the second supply wire and said
second resistance and controlled by a second control current;
first and second constant current generators coupled to said second
resistance and producing first and second constant currents having an
equal value;
a first control transistor, controlled by said output voltage signal and
coupled said first constant current generator, said first control
transistor conducting a first current from said first constant current
generators to the second supply wire, the node coupling said first
constant current generator to said first control transistor also producing
said first control current; and
a second control transistor, controlled by said output voltage signal and
coupled said second constant current generator, said second control
transistor conducting a second control current from said second constant
current generator to the second supply wire, the node coupling said second
constant current generator to sad second control transistor also producing
said second control current, said second current being greater than said
first current, whereby said first output transistor is conducting and said
second output transistor is cutoff if the voltage on the first supply wire
is above a predetermined threshold, and said first output transistor is
cutoff and said second output transistor is conducting if the voltage on
the first supply wire is below said predetermined threshold.
16. The circuit of claim 15 wherein said second constant current has a
value equal to the sum of said first constant current and the maximum
value for said first control current.
17. The circuit of claim 15 wherein said first output transistor operates
in a liner mode of operation if the voltage on the first supply wire is
above a second threshold.
18. The circuit of claim 15 wherein said capacitor charges with a current
whose value is determined by the ratio between said first and second
resistances.
19. The circuit of claim 17 wherein said first output transistor operates
in a saturated mode of operation if the voltage on the first supply wire
is below said second threshold.
20. The circuit of claim 19 wherein said capacitor charges with a current
determined by the current conducted by said first output transistor and is
proportional to the ratio between said first and second resistors.
Description
TECHNICAL FIELD
The present invention relates to the electrical supply of user's telephone
circuits and, more particularly, to a circuit device for deriving a supply
voltage from the user's line.
BACKGROUND OF THE INVENTION
As is known, the speech circuit of an electronic telephone apparatus is
supplied with electrical energy provided on the user's line from the
so-called central battery. A suitable circuit is arranged to extract the
necessary electrical energy from the line, to provide it at a stabilized
voltage to the speech circuit to be supplied.
A known circuit of this type, which is shown in the block diagram of FIG. 1
of the drawing accompanying the present specification, comprises a
capacitor C which is connected on one side to one wire of the line loop,
indicated with the earth symbol, and, on the other, to the other wire L of
line loop, via the series connection of a resistor R2, the
collector-emitter path of a transistor T and a diode D connected in a
forward biased sense. In reality, as is known, both the earth terminal and
the wire L are connected to the wires of the line loop by other
components, not shown in order not to excessively complicate the drawing.
The transistor T is held in conduction in the saturation region by the
output of an operational amplifier A the inverting input terminal of which
is connected to the connection node between the resistor R2 and the
emitter of the transistor T, and the non-inverting input of which is also
connected to the line L via a second resistor R1. The connection between
the emitter of transistor T and the inverting input of the operational
amplifier A constitutes a negative feedback loop which tends to maintain
the voltage drops on the resistors R1 and R2 and therefore the voltages on
the input terminals of the amplifier equal to one another. Between the
non-inverting input of the amplifier and earth there is shown a current
generator G to indicate that a control current Ic, proportional to the
current flowing in the line and derived from known means, not shown, is
applied to the non-inverting input of the amplifier A. A regulation
circuit SR, connecting between the collector of the transistor T and
earth, acts to stabilize the voltage VS across the capacitor C. The
voltage Vs is applied to the speech circuit, not shown, in order to
constitute its supply.
The maximum obtainable value for the stabilized voltage obviously depends
primarily on the value of the voltage on the line at the connection point
with the user's apparatus, which is given by the voltage of the central
battery reduced by the voltage drop on the line due to the resistance of
the line itself and by the so-called "voltage loss" that is to say the
voltage drop along the path between this connection point and the
effective stabilized voltage take-off point.
In the design of telephone networks it is necessary to arrange that the
user's line does not reach a length such that the voltage at the
connection points to the user's telephone apparatus is less than a minimum
predetermined value which still allows the electrical energy to be taken
from the line at a sufficiently high voltage to supply the speech circuit.
The stabilized supply voltage VS is given by the DC voltage VL of the line
at the connection point of the user's apparatus reduced by the "voltage
loss". For the above described known circuit the "voltage loss" is equal
to the sum of the voltage drop VR1 on the resistor R2, the
collector-emitter voltage V.sub.CEsat of the transistor T in saturation
and the voltage VD of the forward biased diode D.
The stabilized voltage is, therefore:
VS=VL-(VR2+V.sub.CEsat +VD)
Since, typically, V.sub.CEsat and VD are both about 0.3 V and, at the
usually operating currents, VR2 for a resistor of 10 ohms, is about 0.1 V
the overall voltage drop is about 0.7 V. Now, the stabilized voltage
necessary for supplying a normal speech circuit is about 3 V, and
therefore the voltage at the connection point of the resistors R1 and R2
to the line cannot be less than 3.7 V. It is obvious that this puts a
limit on the maximum length of the user's line.
SUMMARY OF THE INVENTION
The general object of the present invention is to move this limit in such a
way as to allow the use of longer user lines.
More particularly, the object of the present invention is to provide a
circuit arrangement for supplying a speech circuit of a user's telephone
apparatus which has an extremely low voltage loss.
This object is achieved by the circuit arrangement defined and
characterized in the claims which conclude the present description.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following detailed
description of a simplified embodiment which is therefore in no way
limitative, given in relation to the drawing, in which:
FIG. 1 represents a known circuit described above, and
FIG. 2 represents a circuit arrangement according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 2, where the same symbols as those used in FIG. 1 refer to
equivalent components or quantities, the wires of the line loop are
connected, in this case too, by means of the usual coupling devices, one
to the point indicated L and the other to the common terminal represented
by the earth symbol. Two resistors R1 and R2 are connected each by one
terminal to the point L and by the other to the non-inverting and
inverting terminals respectively of an operational amplifier indicated A.
The resistor R1 is also connected to a generator G which provides a
control current Ic as for the known circuit of FIG. 1. The output of the
operational amplifier A is connected to the gate terminals of two
identical FET transistors, that is to say field-effect transistors, M1 and
M2. These have their source terminals connected to earth and their drain
terminals indicated D1 and D2 respectively, connected each to a current
generator circuit arrangement CC1 and CC2 respectively, and to the base of
a bipolar transistor, respectively P1 and P2. The two transistors P1 and
P2 have their emitter terminals connected together to the circuit node
between the resistor R2 and the inverting input of the amplifier A and
their collector terminals connected, respectively, to one terminal of a
capacitor C and to earth. The other terminal of the capacitor is also
connected to earth. It is noted that, advantageously, each of the
transistors P1 and P2 can be replaced by a pair of transistors connected
in a Darlington configuration and each of the transistors M1 and M2 can be
replaced by two transistors connected together in cascode, without
changing the circuit or its principle of operation. Across the terminals
of the capacitor C, which constitute the output terminals of the circuit
arrangement, from which a stabilized voltage VS is taken, is connected a
branch voltage stabilizer SR.
The circuit arrangements CC1 and CC2 are constituted by two pairs of
bipolar transistors connected in a current mirror configuration and having
one branch connected to a current generator, respectively G1 and G2, and
the other branch to the drain terminal D1, D2 of the respective transistor
M1, M2.
The operation of the circuit device of FIG. 2 is now considered.
The transistors M1 and M2, being equal and having their source and gate
terminals in common, are always traversed by the same currents. The two
current mirrors CC1 and CC2 have a current gain equal to one, that is to
say they produce an output current I1 and I2 equal to the reference (G1
and G2 respectively), if the respective output transistors are in their
linear region that is to say if the voltage at the nodes D1 and D2 is at
least V.sub.CEsat lower than the voltage of the emitters of P1 and P2. The
current G2 is chosen to be greater than the sum of G1 plus the maximum
base current of P1. Since M1 and M2 must carry the same current the node
D2 must carry a voltage sufficiently high with respect to earth to bring
the output transistor of the mirror CC2 into saturation. In these
conditions the transistor P2 is switched off. At the same time the node D1
must carry a sufficiently low voltage to bring P1 into conduction. In
particular, the circuit parameters are chosen in such a way that, when the
line voltage in the absence of a signal is high, that is to say when the
user's line is relatively short, P1 is in a linear zone and, when the line
voltage in the absence of a signal is the minimum allowed, that is to say
when the line is very long, P1 is polarized in the saturation region.
Clearly, in each case, the capacitor C can be charged through the
transistor P1 with a current gIc where g indicates the ratio between R1
and R2, typically lying between 60 and 200.
While the line voltage or, better, the voltage between the point L and
earth, is greater than or equal to the sum of the stabilized voltage VS,
the voltage V.sub.CEsat of P1 and the drop on R2, VR2, P1 remains in
conduction and P2 remains switched off.
During a negative half-wave of an alternating current signal the
emitter-collector voltage of P1 falls. If the negative half wave is
sufficiently great this voltage can fall to zero and then become negative.
The reduction in the emitter-collector voltage produces a reduction in the
collector current of P1. Consequently, the drop VR2 on the resistor R2
falls, causing a corresponding increase in the output voltage of the
operational amplifier A and an increase in the equal drain currents
I.sub.D of M1 and M2. When the drain current I.sub.D of M2 is equal to I2,
which is greater than I1, the transistor P2 begins to conduct because the
voltage on drain D2 of M2 falls to cause forward conduction of its
base-emitter junction, whilst the transistor P1 remains turned off. This
situation is maintained for the whole of the time in which the negative
half-wave maintains the voltage VL lower than the above mentioned sum.
As soon as the voltage VL rises above this value the voltage drop VR2
reduces, P1 commences to conduct, causing an increase in the voltage drop
across R2. The greater drop on VR2 also causes a reduction in the output
voltage of the operational amplifier A and therefore a reduction in the
drain currents I.sub.D of M1 and M2. As soon as the current I.sub.D
becomes less than the current I2 the voltage on D2 returns to be very
close to the voltage of the emitter of P2 so that this latter transistor
ceases to conduct. The initial conditions are thus resumed.
The voltage across the terminals of capacitor C stabilized by the regulator
SR is applied to the speech circuit.
It is clear how the circuit arrangement described above makes it possible
to achieve the object of the invention. In fact, the "voltage loss" is
limited to the sum of the voltage drop on the resistor R2 and the voltage
V.sub.CEsat of the transistor P1 , that is to say the "voltage loss"
reduced by a voltage drop on a forward biased diode.
With a suitable design of the generators CC1 and CC2, that is to say by
causing that the current I2 is very much greater than the current I1, it
is possible to set a heavy saturation of P1 and therefore a V.sub.CEsat
even smaller. In summary, then, in equal conditions there is obtained a
lower "voltage loss" with respect to the known art of 300-500 mV, which
makes it possible to utilize considerably longer user lines.
From the single embodiment of the invention which has been illustrated and
described it is clear that many variations and modifications are possible
within the ambit of the same inventive concept. For example, rather than
utilizing identical transistors M1 and M2 and different currents I1 and I2
it would be possible to utilize structurally different transistors M1 and
M2 and identical currents I1 and I2 as long as the operating
characteristics of the two transistors are such as to control the
transistors Pl and P2 in the same way as described above; moreover, in
place of the field-effect transistors M1 and M2 it would be possible to
utilize transistor devices or equivalent circuit structures formed by
bipolar transistors and, similarly, in place of the transistors, or
Darlington pairs P1 and P2 it would be possible to utilize transistor
arrangements or equivalent circuit structures formed by field-effect
transistors.
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